Electrostatic biasing of the ALT-II pump limiter

Abstract

Electrostatic biasing experiments using the Advanced Limiter Test (ALT-II) pump limiter in the TEXTOR tokamak have been carried out with the dual goals of: (a) improving the core plasma confinement in the tokamak and (b) enhancing the performance of the pump limiter. The fully toroidal belt limiter has been biased during both ohmic and neutral beam heated discharges. Both polarities of bias have been applied up to a maximum of +or-500 V with no evidence of impurity accumulation in the central plasma, although applying either polarity of bias to the limiter increases recycling from both the limiter face and the vacuum vessel liner. This in turn results in an increase of the central density. The application of a negative bias to the limiter produces a barrier to radial particle transport in the region between the limiter and the wall. This barrier is not observed in either the no bias or the positive bias case. Neither polarity of limiter bias affects the central plasma energy confinement, apparently because the electric field structure producing the radial barrier is outside the limiter tangency radius. The enhanced recycling, coupled with high edge density, increases the radiated power from the plasma edge and may lower the power flux to the plasma facing surface of the limiter blade. In the case of positive limiter biasing, the pressure in the pumped plasma collection scoops of the limiter increases by approximately 20%, corresponding to a similar increase in the particle removal rare of the pump limiter. The increase in the particle removal rate appears to result from a lower edge electron temperature. This is consistent with the observation of an increase in edge radiated power